If an index.html file exists on the server, temporarily rename it and then check.
- Any recent changes to .htacess?
- Any recent changes to mods? (new installs or uninstalls)
- Has your host recently updated the server? (PHP version etc)
- I see vBSEO listed at top left in the drop-downs, is it still installed? If yes it could be hacked (it's vulnerable now, the company and product is gone and it's not supported or secure) you need to consider replacing it with DBSEO or uninstalling and going with mod rewrite friendly url structure.
Edit:
http://www.staffs-vw.co.uk/forum.php is loading fine, I bet it's a index.html file loading before index.php as the server is defaulted to that load order.